KUALA LUMPUR, Malaysia -- Planes and ships from across Asia resumed the hunt Sunday for a Malaysian jetliner missing with 239 people on board for more than 24 hours, while Malaysian aviation authorities investigated how two passengers were apparently able to get on the aircraft using stolen passports.

There was still no confirmed sighting of wreckage from the Boeing 777 in the seas between Malaysia and Vietnam where it vanished from screens early Saturday morning en route to Beijing from Kuala Lumpur. The weather was fine, the plane was already cruising and the pilots had no time to send a distress signal -- unusual circumstances for a modern jetliner to crash.

Li Jiaxiang, administrator of the Civil Aviation Administration of China, said some debris had been spotted, but it was unclear whether it came from the plane. Vietnamese authorities said they had seen nothing close to two large oil slicks they saw Saturday and said might be from the missing plane.

Malaysia's civil aviation chief Azaharuddin Abdul Rahman said his country had expanded its area of operation to the west coast of peninsular Malaysia, on the other side of the country from where the plane disappeared. "This is standard procedure. If we can't find it here, we go to other places," he said.

Finding traces of an aircraft that crashes over sea can take days or longer, even with a sustained search effort. Depending on the circumstances of the crash, wreckage can be scattered over many square kilometers (miles). If the plane enters the water before breaking up, there can be relatively little debris.

Investigators will need access to the flight data recorders to determine what happened. Terrorism is always considered a possibility, but the sudden disappearance of Flight MH370 has given extra emphasis to speculation a bomb might have been on board.

On Saturday, foreign ministries in Italy and Austria said the names of two citizens listed on the flight's manifest matched the names on two passports reported stolen in Thailand. It's unclear how common it is for people to get on flights with fake passports, but the news added to fears of terrorism.

Azaharuddin said Sunday that authorities were "aware of the situation and we are doing an investigation at the moment."

Just 9 percent of fatal accidents happen when a plane is at cruising altitude, according to a statistical summary of commercial jet accidents done by Boeing. Malaysia Airlines CEO Ahmad Jauhari Yahya said Saturday there was no indication the pilots had sent a distress signal.

The lack of a radio call "suggests something very sudden and very violent happened," said William Waldock, who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.

The plane was last inspected 10 days ago and found to be "in proper condition," Ignatius Ong, CEO of Malaysia Airlines subsidiary Firefly airlines, said at a news conference.

Two-thirds of the jet's passengers were from China. The rest were from elsewhere in Asia, North America and Europe.

Asked whether terrorism was suspected, Malaysian Prime Minister Najib Razak said authorities were "looking at all possibilities, but it is too early to make any conclusive remarks."

The South China Sea is a tense region with competing territorial claims that have led to several low-level conflicts, particularly between China and the Philippines. That antipathy briefly faded Saturday as China, the Philippines, Vietnam, Singapore and Malaysia all sent ships and planes to the region.

Malaysia had dispatched 15 planes and nine ships to the area. The U.S. Navy was sending a warship and a surveillance plane, while Singapore said it would send a submarine and a plane. China and Vietnam also sent aircraft to help in the search.

Malaysia Airlines has a good safety record, as does the 777, which had not had a fatal crash in its 19-year history until an Asiana Airlines plane crashed last July in San Francisco, killing three passengers, all teenagers from China.